Polymers and numerous additives are typically compounded into formulations which are then totally cross-linked for enhanced strength properties of the finished article. The starting polymer, prior to cross-linking, must have adequate performance properties such that it may be formulated or compounded with various additives and still maintain processability. For example, in a wire and cable coating operation, the composition must have “green strength”, also known as “melt strength”, to remain on the wire after coating, and not sag or deform on the wire until the composition is cured. Otherwise the wire will have thin spots and the insulating value of the composition is lost. The composition must also undergo a final cure step and achieve good physical properties, such as tensile strength, elongation, and 100% modulus (stress at 100% strain). Typical curing occurs through use of peroxide or irradiation, and for polyethylene in general, the curing through crosslinking phenomenon is well documented (see, for example, Radiation Effects in Materials, A. Charlesby, editor, Pergamon Press, 1960). Polyethylene, especially heterogeneous linear low density polyethylene (LLDPE), when exposed to peroxide and/or radiation under proper conditions, forms gels as the molecular weight builds.
When using a profile extrusion process, a manufacturer usually desires an elastomer that “shear thins” or decreases in viscosity with applied shear forces. Because pressure drop across an extruder die and amperage required to turn an extruder screw are directly related to elastomer viscosity, a reduction in elastomer viscosity due to shear thinning necessarily leads to a lower pressure drop and a lower amperage requirement. The manufacturer can then increase extruder screw speed until reaching a limit imposed by amperage or pressure drop. The increased screw speed translates to an increase in extruder output. An increase in shear thinning also delays onset of surface melt fracture, a phenomenon that otherwise limits extruder output. Surface melt fracture is usually considered a quality defect and manufacturers typically limit extruder output and suffer a productivity loss to reach a rate of production that substantially eliminates surface melt fracture.
When producing profile extrusions with thin walls and a complex geometry, a manufacturer looks for an elastomer with high melt strength (“MS”) and rapid solidification upon cooling in addition to good shear thinning behavior. A combination of a high MS and rapid solidification upon cooling allows a part to be extruded hot and cooled below the elastomer's solidification temperature before gravity and extrusion forces lead to shape distortion. Ultimately, for broad market acceptance, a finished part should also retain its shape despite short term exposure to an elevated temperature during processing, shipping or eventual use.
In spite of the advances made in the art, there is a continuing need for a polyolefin composition that has a relatively high melt strength and also exhibits the requisite shear shinning and processibility for many applications.